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The level of stem cell proliferation must be tightly controlled for proper development and tissue homeostasis. Multiple levels of gene regulation are often employed to regulate stem cell proliferation to ensure that the amount of proliferation is aligned with the needs of the tissue. I focus on proteasome-mediated protein degradation as a means of regulating the activities of proteins involved in controlling the stem cell proliferative fate in the C. elegans germline. Previously, RFP-1 (RING finger protein), a putative E3 ubiquitin ligase was identified as being involved in regulating the proliferative fate. I demonstrated that RFP-1 binds to and causes proteasome-mediated degradation of MRG-1 (mortality related gene), a homologue of the mammalian chromodomain-containing protein MRG15 (MORF4L1), which has been implicated in promoting proliferation of neural precursor cells. I have demonstrated that C. elegans with reduced proteasome activity, or that lack RFP-1 expression, have increased levels of MRG-1 and cause a shift towards increased proliferation in sensitized genetic backgrounds. Likewise, reduction of MRG-1 partially suppresses stem cell over-proliferation. Analysis in tissue culture cells further supports that RFP-1 ubiquitinates and directly targets MRG-1 for degradation by the proteasome. I discovered that MRG-1 levels are controlled independently of the spatially regulated GLP-1/Notch signaling pathway, which is the primary signal controlling the extent of stem cell proliferation in the C. elegans germline. Therefore, I have identified MRG-1 as a key player in regulating the proliferation versus differentiation decision and determined that its activity is controlled, at least in part, through proteasomal degradation.
Furthermore in a second project, using Stable Isotope Labeling of Amino Acids in C. elegans (SILAC) proteomics, I have identified proteins in the C. elegans germline and quantified the abundance of proteins that are enriched in the stem cell population. I have identified eight putative proliferation-promoting proteins that cause suppression of proliferation when their function is reduced.
Stem cells contain the therapeutic power to rejuvenate human life. However, this is possible only if we can clearly understand their behaviour. Through this kind of research we can obtain deeper insight into the functioning of stem cells, necessary to harness their untapped potential.